http error [error code here, eg. 502]

Try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue (theme functions can interfere like plugins).

Also see: https://www.wpbeginner.com/common-wordpress-errors-and-how-to-fix-them/#502badgateway

If the above steps do not resolve the issue, try contacting your hosting provider.